The Department of Defense, also known as the Pentagon after the building in which it is headquartered, is the federal agency responsible for national security and the United States armed forces. It is the largest employer in the world, claiming more than 3 million workers; the Pentagon, in turn, is the largest office building in the world. The Department of Defense is also the federal agency with the most budgetary resources, and regularly accounts for more than half of the United States federal discretionary budget. In 2007, EFF sued the Department of Defense for information about its use of national security letters to gather intelligence within the U.S.